Since most of oil or natural gas produced in an oil well contains wet carbon dioxide (CO2), either an inhibiter is used in a carbon steel or a martensitic stainless steel containing 13% Cr is employed in order to protect the corrosion of either oil country tubular goods, such as tubing used for drilling and production of an oil well, or line pipes used for transportation. In particular, 13% Cr steel is widely used, because it has a good corrosion resistance in an environment containing wet carbon dioxide and steadily provides high mechanical strength. However, it is known that the 13% Cr steel often provides sulfide stress cracking when used in an environment containing hydrogen sulfide (H2S), thereby causing its usage to be restricted.
In recent years, the environment of an oil well from which oil or natural gas is produced increasingly has become severe. Most of the oil well containing carbon dioxide contains a very small amount of hydrogen sulfide. Even in an oil well containing only carbon dioxide in the initial stage, a very small amount of hydrogen sulfide is generated little by little as it is used. In this case, moreover, a problem has to be taken for corrosion resulting from a fluid flowing at a high speed, i.e., a corrosive wear.
It is empirically recognized that the restriction of the highest hardness is effective to reduce the sensitivity to sulfide stress cracking of 13% Cr steel. For instance, in NACE MR0175, the highest hardness has been specified so as to be restricted to 22 in HRC (Rockwell hardness in scale C), when 13% Cr steel, e.g., SUS 420 steel is used in a corrosive environment containing hydrogen sulfide.
Recently, the above 13% Cr steel has been improved so as to be used in a much severer corrosive environment, so that an improved type 13% Cr steel containing an extremely small amount of carbon and an appropriate amount of nickel in spite thereof has been developed. Even in this steel, the highest hardness is restricted to 27 in HRC (see NACE MR0175-2001).
With regard to the above-mentioned improved type 13% Cr steel, several steels having a high mechanical strength and an excellent corrosion resistance have been proposed. For instance, in Japanese Patent Application Laid-open No. 2-243740, a martensitic stainless steel having a high mechanical strength and an excellent corrosion resistance even in the state of being either hot worked or quenched is disclosed, in which case, not only Ni but also Mo is added thereto. Moreover, in Japanese Patent Application Laid-open No. 2-247360, a martensitic stainless steel having a high mechanical strength, together with excellent corrosion resistance in carbon dioxide environment and excellent stress corrosion cracking resistance, has been proposed, where a specific amount of Cu is contained in the 13% Cr steel.
The proposed steels pertain to 13% Cr steel having a specified magnitude for the highest hardness as well as a high mechanical strength and excellent corrosion resistance, and these steels further have an excellent corrosion resistance in a corrosive environment containing carbon dioxide and a very small amount of hydrogen sulfide. Nevertheless, the resistance to the corrosive wear cannot be obtained with these steels.
In other words, the steel has to satisfy both the corrosion resistance in carbon dioxide and the sulfide stress cracking resistance in order to ensure the resistance to corrosive wear in a very severe oil well environment, and the steel also has to increase the hardness in order to enhance the resistance to corrosive wear. However, the 13% Cr steel having a restricted magnitude in the highest hardness can hardly satisfy the resistance to corrosive wear in an increasing severity of oil well environment.
On the other hand, a technology capable of enhancing the resistance to corrosive wear in a martensitic stainless steel is disclosed. In Japanese Patent Application Laid-open No. 6-264192 and No. 7-118734, martensitic stainless steels having a high mechanical strength and excellent resistance to corrosive wear are described, where nickel is added in a high content to the 13% Cr steels. These steels are normally used in a steel material or a welded structure having a high mechanical strength, wherein it is important to suppress the cavitation-erosion resulting from cavities in a hydrofoil or a facility of sand drainage. However, these steels are not useful for using in an environment of corrosive wear due to the fluid flown at a high speed in a corrosive environment.